When you think of contaminants in water, if you believed they simply came from the source, you’d be mistaken; new data from researchers at the University of Michigan shows that most bacteria found in drinking water actually comes from the filters of water treatment plants.

The study, unique in its own right, took a broad look at the water supply in Ann Arbor, from source to tap. What researchers found is that most of the bacteria in water coming out of the tap did not come from the aquifers or rivers where it originated, but from filters at treatment plants designed to remove such bacteria. Researchers hope their findings lead to more sustainable water treatment processes that utilize fewer chemicals and result in tap water containing lower levels of byproducts that pose public health risks. They say their work could eventually provide engineers the ability to control the types of microbes in drinking water, thereby improving overall health like “live and active cultures” in yogurt.

The study, led by Lutgarde Raskin, a professor of civil and environmental engineering and which is published online in Environmental Science & Technology, spanned six months in which researchers took samples of drinking water at 20 points along its path from groundwater and Barton Pond sources, to a number of places along the water treatment cycle, and ending at tap sources. The team harvested bacteria from each sample then sequenced the bacterial DNA.

The researchers noted that tap water is loaded with bacteria, despite intensive filtering and disinfection processes from source to sink that occur in most of the developed world. But they add that’s not necessarily problematic, and in fact could serve as an opportunity to improve the process overall.

Perhaps some good can come of this

“A major goal right now in drinking water treatment is to kill all bacteria because there’s the perception that all bacteria are bad,” said Ameet Pinto, a professor at the University of Glasgow who assisted Raskin in the project as part of post-doctoral research. “But there’s a good bit of scientific literature that says there are good bacteria, innocuous bacteria and bad bacteria.

“If we can better understand the types of bacteria in the microbial community from source to tap and what processes control it, perhaps we can be more effective at controlling which ones get through,” he added.

A d v e r t i s e m e n t

Much of the prior research that focused on drinking water concentrated on pathogens which cause disease, said Pinto. Legionella, Salmonella and E. coli bacteria don’t exist in isolation, he said; their fate is largely dependent on the microbial community that surrounds them.

“The more critical questions are, ‘Where do they come from’ and, ‘What determines which ones survive treatment and end up in our drinking water?’ These questions have not been systematically asked until now,” Pinto said.

Disinfection can be harmful too

Researchers found that “activated carbon filters” most common to the filtration process were dominant in determining which bacteria became most prevalent in tap water. Raskin’s team found that, for example, the level of Alphaproteobacteria was found to be about six percent in drinking water but 38 percent on the filters and 23 percent of the bacterial community at the tap source.

This pattern repeated itself time and again, despite regular filter cleaning. These bacteria, which are mostly harmless, were able to form biofilms on filters then slough off into the water and survive the resultant disinfection process.

Treatment plants typically add chemicals like chlorine to drinking water, but such disinfectants can react with substances that occur naturally in the water to form harmful byproducts, according to the Environmental Protection Agency, which sets “legally enforceable standards that apply to public water systems,” the agency says on its website.

“Disinfection can form harmful chemicals in drinking water,” said Chuanwu Xi, an associate professor of environmental health sciences at the School of Public Health, who assisted Raskin.

“If we can get away with not using so many chemicals and prevent the formation of these byproducts, we should think about limiting their use,” he said.